Heretofore, sodium hydrogencarbonate (NaHCO3 which is also called baking soda or sodium bicarbonate) is widely used in the field of various food products, as baking powder, an additive for soft drinks, etc., in the pharmaceutical industry as a dialysate, an antacid, etc., and further as a fire-extinguishing agent, as a bath additive, as a detergent, as a blasting medium, as an acidic gas neutralizing agent, etc. In most cases, such sodium hydrogencarbonate is produced, transported, stored, sold and used in the form of powdery or granular crystal particles.
However, commercial crystal particles of sodium hydrogencarbonate generally show a caking property and is empirically known to have a caking property resulting from a very small amount of anhydrous sodium carbonate, sodium carbonate monohydrate or Wegscheider's salt, formed on the surface of the sodium hydrogencarbonate crystal particles by drying in the production process, especially in a storage environment with a high humidity such as in the rainy season. If caking occurs, the flowability of the particles will be low, and the handling efficiency will deteriorate remarkably in the respective steps from distribution to use, and various troubles are likely to be brought about. Thus, the caking is a serious problem which may impair the commercial value of sodium hydrogencarbonate.
However, heretofore, it has been very difficult to quantitatively determine whether the sodium hydrogencarbonate crystal particles are practically caked during storage or not in a short period of time with high reproducibility. For example, Patent Document 1 discloses a test for evaluating the caking property by storing a sample of sodium hydrogencarbonate crystal particles at a temperature of 30° C. at a relative humidity of 80% for 1 week. However, under such conditions, the period is short and the caking phenomenon which will occur in the rainy season is not accurately reproduced, and this test is inappropriate as an evaluation method. Further, the method is not a quantitative evaluation method wherein the mass or the volume is weighed, and the method is not to package a certain amount, whereby agglomerates resulting from the caking can not be quantitatively measured, such being poor in reproducibility.
Further, Patent Document 2 discloses a test for evaluating the caking which comprises storing a sample of sodium hydrogencarbonate crystal particles at a temperature of 30° C. at a relative humidity of 80% for one day, storing it at a temperature of 20° C. at a relative humidity of 50% for one day in addition, and repeatedly carrying out these storages alternately for three weeks. However, in such an evaluation method under these conditions also, the caking phenomenon which will occur in the rainy season is not practically reproduced, similar to the Patent Document 1, and this test is inappropriate as an evaluation method. For example, even when the change in the temperature between 30° C. and 20° C. is considered as a difference between day and night, as the relative humidity in the interior of the bag will increase when the temperature is decreased at night, the test conditions are opposite to a phenomenon which practically occurs. Further, the material or the laminate structure of the packaging material is also not disclosed. Therefore, accurate reproduction is impossible.
Patent Document 3 discloses an evaluation method under such conditions that five packaging bags each having 25 kg of sodium hydrogencarbonate crystal particles packed therein are piled up and stored at 25° C. for 4 weeks under pressure, but it does not disclose the relative humidity. Further, Patent Document 4 discloses a method for evaluating the caking property under similar conditions. These conventional tests for evaluating the caking, evaluation conditions of which are not sufficiently disclosed, or the evaluation method of which is not a quantitative method, are poor in reproducibility.
As mentioned above, in a case where the test for evaluating the caking property is poor in the reproducibility and is poor as a quantitative method, the results of the evaluation test are poorly reliable and further, in a case where there are two or more samples to be compared, the test for evaluating the caking property should be simultaneously conducted. That is, it is difficult to compare results of the tests for evaluating the caking property carried out in separate times at separate locations, and the results of the caking property evaluation test have very low versatility for the later utilization of the data.
Further, with respect to pharmaceuticals, Guideline for Stability Testing of New Drug Substances and Products by Ministry of Health, Labour and Welfare (PMSB/ELD Notification No. 0603001) may be mentioned. Conditions for stability evaluation employed here are such that the temperature is 25° C. or 30° C. and the relative humidity is 60%, and the temperature is 40° C. and the relative humidity is 75%. Under these conditions, if employed as conditions for the method for evaluating the caking property of sodium hydrogencarbonate crystal particles, the period is so long as 12 months or 6 months, and the evaluation takes too long, and a temperature of 40° C. corresponds to a condition under which sodium hydrogencarbonate itself is likely to be decomposed, and such conditions are significantly different from a state in which caking practically occurs. Thus, no results of tests on caking with high reproducibility will be obtained in a short period of time.    Patent Document 1: Japanese Patent No. 3306873    Patent Document 2: JP-A-2003-104722    Patent Document 3: JP-A-2004-2166    Patent Document 4: JP-A-2004-203673